Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
PCGen is a Java (1.5+) -based character generator aimed at supporting OGL role playing games like those based upon the SRD, RSRD, and MSRD (such as Dungeons and Dragons). It is highly configurable by the users, allowing the Game Master to add or remove races, classes, feats, etc., and to implement house rules such as differing class or race restrictions. Level advancement is also supported, with configurable progressions of feats, skills and ability score bonuses. For "at the table" support, PCGen offers a set of GM plugins. Further options allow the user to produce a customized character sheet, which can then be viewed in, or printed from, the user's WWW browser. It can also produce a "party" sheet with the information needed by the GM.
The Interreality project is a collection of Open Source, Free Software projects based on the Virtual Object System (VOS) platform. The goal is to develop a free and open platform for multi-user 3D virtual reality games, applications, and interactive, collaborative 3D virtual spaces. VOS is a powerful and generic network object framework designed to support a variety of high-performance applications. It is the basis for Ter'Angreal, a general-purpose browser application for entering and participating in 3D worlds.
PLIB is a set of libraries to write games and other realtime interactive applications that are 100% portable across a wide range of hardware and operating systems. It's used by Majik3D, FlightGear, and others, and includes libraries for GUI widgets, sound replay, geometry, scene graph, joystick, and fonts/text.
Anywhere Board Games (ABG) allows users to play board and card games on the Web in real time on multiple machines (PC, Linux, iPhone, iPad, and Android). ABG runs as a Google+ Hangout App or as a standalone PHP app. Unlike other gaming environments, this one has a peculiar vision: projecting the main board on our table and using laptops or tablets to control players' hands (with other friends joining in remotely). It supports flipping, stacking, rotating, rolling, and shuffling of pieces, so most board and card games are playable.